This invention relates to the developing of electrostatic images such as is performed in xerography and ionography, and in particular, to a new and improved fountain for delivering liquid toner to a receptor for developing an electrostatic image into a visible image.
In a typical system, an electrostatic image of varying charge density is produced on a receptor, such as a selenium plate or a sheet of dielectric plastic. Toner particles are deposited on the receptor, with the toner density being proportional to the electrostatic field density normal to the receptor. The toner is then fixed in place, as by heating, and the image is ready for viewing. In some systems, dry toner is used with the charged receptor being exposed to a cloud of the toner particles. In other systems, liquid toner is used with the charged receptor exposed to a liquid having the toner particles suspended therein.
The present invention provides an improved fountain for delivering the liquid toner to the receptor and is particularly useful with systems wherein the latent electrostatic charge density is small thereby requiring the use of a sensitive toner (low charge per particle) and developing system.
An important difference between electrostatic development and conventional silver halide radiographs resides in the so-called edge enhancement characteristic of the electrostatic image which can be used to emphasize small detail. This feature is related to the fact that the deposition of toner at a given point on the latent image, and hence the final optical density, is determined not only by the charge density at that point, but also by the charge at neighboring points. When a charge gradient is present on the latent image in the form of a step, then fringing electrostatic fields are established, which both prevent toner deposition along one side of the edge of the step, and cause additional development along the other side of the step. This "edge effect" is well known and reference should be made to the literature for more information.
In the development of any electrostatic image, image quality is deteriorated whenever toner is deposited without regard to the charge present on the image receptor. This spurious toner deposit decreases the contrast of the visual image and imparts artifacts to the image. Such degradation is particularly evident in systems where the electrostatic charge on the receptor is small, such as in electronradiograms, a form of x-ray ionography. For further information on the basic image forming system, reference may be made to U.S. Pat. No. 3,774,029.
In order to obtain the charge sensitivity necessary to develop the low charge levels present on an electron radiograph, while concurrently meeting the demands of low toner granularity and fast reliable development, it has been found that electrophoretic (liquid) development is required, as opposed to dry, powder cloud development. Liquid toners have been developed to satisfy these requirements. One known apparatus for delivering the liquid toner to the surface of the receptor carrying the latent image consisted of a metal fountain, wherein the toner formed a shallow pool at the top of the fountain, wetting the receptor as it traversed the fountain. The separation between the top of the fountain and the image plane was in the order of 1 mm. Such a design caused the electrostatic fields to be prependicular between the fountain and the receptor, thus eliminating the fringing fields which yield the desired edge enhancement. Deepening the pool does not alleviate this situation as long as the pool bounderies are close to the image, as the development occurs very rapidly, and is concentrated at those electrically conductive areas of the fountain which are closest to the image. Furthermore, as the pool depth is increased, image artifacts due to non-laminar hydrodynamic flow of the toner become more pronounced, particularly at discontinuities in the pool geometry.
It is an object of the present invention to provide a new and improved apparatus for dispensing liquid toner for developing an electrostatic image which allows edge enhanced development to occur while not creating artifacts due to irregular fluid flow. A further object is to provide such a new and improved fountain which can be directly substituted for the prior art fountain in the developer apparatus.